1. Field of the Invention
In microwave FM (frequency modulation) communication systems capable of handling frequency division multiplex telephone channels or video signals, the noise generated by local oscillators is always an important system consideration. To satisfy the overall noise performance requirements, in a multihop microwave system the local oscillators must meet relatively stringent noise limits. For example, the FM noise of a single local oscillator in the baseband frequency range 70 kHz - 4MHz typically should be more than 83 dB below the standard 200 kHz rms FM deviation. Such noise requirements as this dictate the use of particular types of oscillator designs and preclude the use of others.
2. Description of the Prior Art
A number of different microwave signal generators suitable for use as local oscillators have been designed for good short-term stability and low FM noise generation. A survey of such oscillators and the problems encountered with these designs is found in a article entitled, "Short Term Stable Microwave Sources", by D. B. Leeson, in The Microwave Journal, June 1970.
One of the most common type of microwave local oscillator is known as the crystal-multiplier microwave source. A quartz crystal, placed in a VHF oscillator, is usually used as the basic frequency reference element. Frequency multipliers, usually using varactors or step recovery diodes, increase the frequency of the VHF crystal oscillator to the desired microwave frequency. Amplifiers are also used in the oscillator-multiplier chain to overcome the high power losses of the multipliers. A bandpass filter selects the desired frequency component and suppresses the unwanted harmonics. Incidental to this design is a microwave isolator which matches the multiplier to the filter, and thereby reduces reflections back into the multiplier stage.
Although this design is seemingly simple, the actual circuit can be quite complex. Furthermore, FM noise is often a problem in this design because of the high level of amplification needed to drive the multiplier circuits. Typically, a class C amplifier design is used which produces AM and PM signal distortion and shot noise. And when this noise is applied to the frequency multiplier, the noise frequency deviation is amplified by frequency multiplier factor. Consequently, a noise stripping filter is commonly used at the amplifier output to reduce the FM noise. Such filters add to the oscillator cost, and they have their own design deficiencies, such as frequency stability. Furthermore, power dissipation in this design is high whether the amplifier is a class A or a class C type design. High power dissipation naturally leads to heat dissipation problems.